This invention relates to magnetic particles having a glass surface and are substantially spherical. This invention also relates to methods for making them, as well as to suspensions thereof and their uses for the purification of biological material in particular in automated processes.
Many biological materials, especially nucleic acids, present special challenges in terms of isolating them from their natural environment. On the one hand, they are often present in very small concentrations and, on the other hand, they are often found in the presence of many other solid and dissolved substances that make them difficult to isolate or to measure, in particular in biospecific assays.
Biospecific binding assays allow of the detection of specific analytes, e.g. nucleic acids, or specific analyte properties and play a major role in the field of diagnostics and bioanalytics. Examples therefor are hybridisation assays, immuno assays and receptor-ligand assays.
Hybridisation assays make use of the specific base-pairing for the molecular detection of nucleic acid analytes e.g. RNA and DNA. Hence, oligonucleotide probes with a length of 18 to 20 nucleotides may enable the specific recognition of a selected sequence in the human genome. Another assay which makes use of the selective binding of two oligonucleotide primers is the polymerase chain reaction (PCR) described in U.S. Pat. No. 4,683,195. This method makes use of the selective amplification of a specific nucleic acid region to detectable levels by a thermostable polymerase in the presence of desoxynucleotide triphosphates in several cycles.
Nucleic acids are comparatively complex analytes which have normally to be extracted from complex mixtures before they can be used in a probe-based assay.
There are several methods for the extraction of nucleic acids:                sequence-dependent or biospecific methods as e.g.:                    affinity chromatography            hybridisation to immobilised probes on beads                        sequence-independent or physico-chemical methods as e.g.:                    liquid-liquid extraction with e.g. phenol-chloroform            precipitation with e.g. pure ethanol            extraction with filter paper            extraction with micelle-forming agents as cetyl-trimethyl-ammonium-bromide            binding to immobilised, intercalating dyes, e.g. acridine derivatives            adsorption to silica gel or diatomic earths            adsorption to magnetic glass particles (MGP) or organo silane particles under chaotropic conditions                        
Many procedures and materials for isolating nucleic acids from their natural environment have been proposed in recent years by the use of their binding behavior to glass surfaces. In Proc. Natl. Acad. USA 76, 615-691 (1979), for instance, a procedure for binding nucleic acids in agarose gels in the presence of sodium iodide in ground flint glass is proposed.
The purification of plasmid DNA from bacteria on glass dust in the presence of sodium perchlorate is described in Anal. Biochem. 121, 382-387 (1982).
In DE-A 37 34 442, the isolation of single-stranded M13 phage DNA on glass fiber filters by precipitating phage particles using acetic acid and lysis of the phage particles with perchlorate is described. The nucleic acids bound to the glass fiber filters are washed and then eluted with a menthol-containing buffer in Tris/EDTA buffer.
A similar procedure for purifying DNA from lambda phages is described in Anal. Biochem. 175, 196-201 (1988).
The procedure known from the prior art entails the selective binding of nucleic acids to glass surfaces in chaotropic salt solutions and separating the nucleic acids from contaminants such as agarose, proteins or cell residue. To separate the glass particles from the contaminants according to the prior art, the particles are either centrifuged or fluids are drawn through glass fiber filters. This is a limiting step, however, that prevents the procedure from being used to process large quantities of samples.
It has been demonstrated that magnetic particles covered with a glass surface offer considerable advantages for isolating biological materials. If the magnetic particles have not been brought in contact with a magnetic field, gravity is the only force that can cause them to sediment out. They can be resuspended by shaking the solution.
The sedimentation procedure that does not utilize a magnetic field proceeds more slowly than the immobilization of biological materials on the surface of the particles. This is especially true for nucleic acids. The magnetic particles can be easily collected at a specific location in the sample fluid by means of a magnet. The fluid is then separated from the particles and, therefore, from the immobilized biological materials. The use of magnetic particles to immobilize nucleic acids after precipitation by adding salt and ethanol is described in Anal. Biochem. 201, 166-169 (1992) and PCT GB 91/00212. In this procedure, the nucleic acids are agglutinated along with the magnetic particles. The agglutinate is separated from the original solvent by applying a magnetic field and performing a wash step. After one wash step, the nucleic acids are dissolved in a Tris buffer. This procedure has a disadvantage, however, in that the precipitation is not selective for nucleic acids. Rather, a variety of solid and dissolved substances are agglutinated as well. As a result, this procedure can not be used to remove significant quantities of any inhibitors of specific enzymatic reactions that may be present. Magnetic, porous glass is also available on the market that contains magnetic particles in a porous, particular glass matrix and is covered with a layer containing streptavidin. This product can be used to isolate biological materials, e.g., proteins or nucleic acids, if they are modified in a complex preparation step so that they bind covalently to biotin.
Magnetizable particular adsorbents proved to be very efficient and suitable for automatic sample preparation. Ferrimagnetic and ferromagnetic as well as superparamagnetic pigments may be used for this purpose.
Particles, according to the expert, are solid materials having a small diameter. Particles like these are often also referred to as pigments.
Those materials are referred to as magnetic that are drawn to a magnet, i.e., ferromagnetic or superparamagnetic materials, for instance. Superparamagnetism is seen as advantageous and preferable in the state of the art (e.g. U.S. Pat. No. 5,928,958; U.S. Pat. No. 5,925,573; EP 757 106). The glass or organosilane surfaces are often functionalised in order to be used for biospecific capture reactions, e.g. U.S. Pat. Nos. 5,928,958, 5,898,071, 5,925,573, EP 937 497, U.S. Pat. Nos. 4,554,088 or 4,910,148. Alternatively, glass or organosilane surfaces may be treated with various solvents or salts to modify their hydrophilicity and/or electropositivity, e.g. U.S. Pat. No. 5,438,127.
However, the underivatized silanol groups of the glass or the silane surface may be used for the adsorption with pure physico-chemical forces under suitable reaction conditions as described in DE 195 20 964, DE 195 37 985, WO 96/41840, WO 96/41811, EP 757 106 or U.S. Pat. No. 5,520,899. Typically, magnetic cores or magnetic core aggregates are covered with a glass surface which is formed by an acid- or base-catalyzed sol-gel-process. These particles are called core-shell particles. The glass shell then has a typical layer thickness (see e.g. DE 195 20 964) wherein the size and shape of the pigment, which may contain a non-magnetic support as e.g. mica in addition to the magnetic metal oxide, determines size and form of the produced particle (see e.g. DE 195 37 985 and corresponding WO 96/41811). To obtain a high surface activity, glass material with a high porosity is used (see e.g. EP 757 106; WO 99/26605). Further, composite magnetic particles are described, e.g. silicate-covered ferric oxide covered with an inorganic silica matrix from silica particles (EP 757 106) or mixtures of glass and silica gel (WO95/06652).